Jump indexing pallet and method for heat exchanger assembly
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for assembling heat exchanger elements and particularly to such a method and apparatus for moving pallets into a loading station to receive such elements. 
     BACKGROUND OF THE INVENTION 
     Heat exchangers of the plate type are comprised of pairs of preformed plates joined to other pairs at their ends by integral bosses and separated at their middle section by air centers or corrugated fins, the plates and fins all being brazed together so that each pair of plates becomes a tube for carrying refrigerant, the bosses serving as a manifold for permitting refrigerant flow from one tube to another, and the fins facilitating heat exchange between the tubes and air flowing outside the tubes. U.S. Pat. No. 4,470,455 issued to Sacca describes such a plate type heat exchanger in detail. 
     The assembly of the plate type heat exchanger elements into a core ready for brazing has typically been carried out largely by hand operations. Specifically, the first step is to assemble a fin element between two plates and crimp the plates together into subassemblies where their bosses connect, and then manually stack such subassemblies along with side plates into a fixture which holds each subassembly in place. 
     It is desirable to enhance the assembly practice by an improved method and machine for assembly. In particular it has been found that the process is improved in terms of automation and in terms of reducing spacing in the fixture if it is begun by joining the plates together into pairs that eventually become tubes and inserting the plate pairs and side plates into a fixture and then inserting the centers between the plates. It is desirable to have a machine to perform the assembly operations to reduce the manufacturing expense and otherwise improve the efficiency of the assembly practice. It has been demonstrated that the machine assembly of plate pairs, air centers and side plates into a pallet is practical. 
     To economically assemble the elements it is desirable to avoid large inventories of parts and any nonessential handling of the parts. Here it is proposed to manufacture at least some of the parts adjacent the loading station and directly feed the parts to the pallet. Thus plate pairs are assembled and immediately dropped into the pallet so that there is no inventory of plate pairs. Also machines for manufacturing the air centers discharge each center directly into the pallet at the loading station so that the centers are inserted into assemblies as fast as they can be produced. This requires the synchronization of the manufacturing apparatus and the assembly apparatus. In particular, the center manufacture is essentially a rolling mill operation which is a continuous process so that parts are made at an optimum rate. While variation of the rate is possible, there are limitations on how fast the rate can be changed so that it is not practical to stop the machine suddenly and restart the machine to accommodate discontinuous assembly procedures. Rather, it is preferred to accommodate the assembly method and apparatus to the center manufacture. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a method and apparatus for quickly advancing a pallet into position for loading. 
     The invention is carried out by a pallet system for the assembly of heat exchanger elements comprising; a plurality of pallets movable through a loading station, lead screw means selectively engagable with the pallets for controlling pallet position, motor means drivingly connected to the lead screw means for advancing each pallet with respect to the loading station, the lead screw means having a first screw means for indexing one pallet through the loading station and for discharging the one pallet from the station and having a second screw means for advancing another pallet to the loading station during the discharge of the said one pallet for operative engagement by the first screw means whereby the rate of movement of the said another pallet is under positive control of the motor means for rapid indexing. 
     The invention is further carried out by the method of sequentially loading a plurality of pallets at a loading station with minimal delay between pallets comprising the steps of: indexing a first pallet through a loading station for loading, engaging a second pallet while the first pallet is loading and indexing the second pallet synchronously with the first pallet, when the first pallet is loaded, positively advancing the first pallet and the second pallet at a rapid rate to simultaneously discharge the first pallet and position the second pallet in the loading station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings wherein like references refer to like parts and wherein: 
     FIG. 1 is an elevation partly in section of stacked plates and air centers for a heat exchanger core, 
     FIG. 2 is an isometric view of an indexed pallet system according to the invention, 
     FIG. 3 is a plan view of the right half of a pallet and associated screw assembly of FIG. 2, 
     FIG. 4 is a side view of the pallet of FIG. 3, 
     FIG. 5 is an end view of the pallet of FIG. 3 with a plate of FIG. 1 loaded into the pallet and with an end plate removed to show block detail, 
     FIG. 6 is a partly broken away end view of the block of FIG. 5, and 
     FIG. 7 is a partial plan view of the dual screw arrangement of FIG. 2 and associated pallets according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A portion of the heat exchanger core to be assembled by the apparatus and method of the invention is shown in FIG. 1. Plate pairs 10 which will form tubes when brazed consist of plates 12 and 14 which when stacked form a heat exchanger core adapted to be used as an evaporator. The plate pairs 10 are stacked to define a space 16 therebetween for the flow of air. The space 16 includes a corrugated metal center 17 with fins struck out therefrom for increasing the heat exchange efficiency. Only a portion of the center 17 is illustrated. 
     For standard plate pairs 10 the individual plates 12 and 14 are configured identically, one of the plates is simply inverted and rotated 180° relative to the other. Each plate has a flat peripheral edge portion 18 and the portions 18 of the two plates are formed so as to engage one another prior to braze jointure. Thus each pair of plates, when brazed, forms a tube for refrigerant. Inlet and outlet manifolds 24, 26 are formed by outwardly offset and generally circular portions 28 in each end of plates 12 and 14. An opening 30 is provided in the top surface at one end and an opening 32 is provided with an outwardly raised flange portion 34 at the other end. Thus, the plates are designed so that, when stacked, the flange portion 34 surrounding opening 32 fittingly engages the opening 30. This provides a registering relationship between the plates of two adjacent tubes. 
     In some cases the plates 12 and 14 are not configured identically in the sense that an opening 30 may be omitted to structure the manifold for fluid flow management through the core. The plate on either end of the core will be equipped with an inlet or outlet fitting at the opening 30 or 32. Thus the core is made up primarily of &#34;standard&#34; plate pairs combined with a few &#34;special&#34; plate pairs. In any event they all couple together in the same manner. The plate and heat exchanger structures are more fully described in the U.S. Pat. No. 4,470,455 to Sacca. 
     The general organization of the assembly machine is shown in FIG. 2. A pallet 40 comprises an open-sided frame 42 with vertical end plates 44 at each corner of a horizontal base plate 45. Four rods 46 supported by the end plates 44 extend longitudinally along each side to pass through and hold a plurality of perforated blocks 48 which can slide a limited amount along the rods. A coil spring 49 under compression surrounds alternate rods 46 on each side between an end plate 44 and the nearest block 48 to hold the blocks together against the other end of the frame unless the spring force is overcome. The blocks 48 each have a slot 50 for receiving the edges of a plate pair 10 adjacent an offset portion 28. Each block also has an outboard cam follower 52 extending to the side of the pallet. A lead screw 54 with its axis parallel to the rods 46 at each side of the path of the pallet engages just a few of the cam followers 52 at a given time. The lead screws 54 are synchronously rotated by servomotors 56 to advance the corresponding blocks 48 longitudinally so as to precisely position the blocks and to advance the entire pallet 40 as well. A microprocessor based controller 57 controls the servomotors. The lead screws are positioned at a loading station for plate pairs 10 and the pallets 40 are carried to the loading station by a power and free conveyor 58 which depends on a frictional contact to drive the pallet. The lead screws engage the cam followers of the blocks and positively and precisely position the blocks at a feed plane where plate pairs are dropped into the slots 50 in the blocks 48. As shown in FIG. 2 the first few blocks are holding plate pairs 10 and subsequent blocks are prepared to receive a plate pair being inserted as indicated by an arrow 60. 
     The width of the blocks is such that when they are nested together the adjacent plate pairs 10 are stacked together as shown in FIG. 1. A critical function of the pallet system is that the pitch of the lead screw 54 is greater than the width of the blocks so that the few blocks that are actively engaged by the lead screws are spaced far enough to permit insertion of the plate pairs 10 without interference by an adjacent plate pair and the adjacent pairs are moved into a nested assembly as they are released by the lead screws. The plate pairs 10 may not necessarily be loaded at the same station since it may be more convenient to have separate loading stations for each type of plate pair, standard or special. The conveyor 58 carries the pallet from one station to the next. At each station the blocks 48 are spaced apart as they pass the feeding plane and the proper plate pairs are inserted into empty slots according to a preset program. Centers 17 are also supplied to the pallet in the same manner. Center insertion occurs after all the plates are inserted since the plate pairs 10 position and laterally support the centers. 
     A center loading station is shown in FIG. 2 downstream from the plate loading station. Lead screws 62 driven by servomotors 64 are on opposite sides of the conveyor 58. The optimum spacing of the blocks for center insertion is less than for the plate pair insertion. Thus the lead screws 62 at the center loading station have a smaller pitch than those at the plate pair loading station. Accordingly, the pallet 40 can provide various insertion spacings under control of the lead screws at a various stations. 
     A special feature at the center loading station is an auxiliary lead screw 66 drivingly coupled to each lead screw 62 by a shaft 68 but spaced from the lead screw 62 in the direction of advancement by a distance of perhaps one half the length of a pallet 40. The purpose of the lead auxiliary screw 66 is to engage a pallet 40&#39; which is waiting to enter the center loading station and positively advance the pallet at a rate determined by the motor 64 speed. In the absence of the lead screw 66 the pallet would be advanced by the power and free conveyor 58 which relies on friction to move the pallet and is accordingly limited in its ability to accelerate the pallet. The positive advancement is most advantageous when the waiting pallet is touching or nearly touching the pallet in the station. By positively advancing the waiting pallet 40&#39; it can be accelerated quickly for positioning in the station under control of the lead screw 62, thus minimizing the time lapse between the last center insertion in one pallet 40 and the first center insertion in the next pallet. The benefit of minimizing the time lapse is to allow the supply of centers to proceed at a more uniform rate. In the most efficient arrangement the centers are fed to the pallet directly from the machine making the centers. That machine operates best at a constant output rate but it can vary its rate somewhat to accommodate the time lapse between pallets, providing that the time lapse is small. In other words, it is not desirable to stop the supply of centers each time a pallet is positioned in the loading station but some slow down is permissible. The positive advancement of the waiting pallet by the lead screw 66 in conjunction with the control by lead screw 62 permits its precise positioning in the loading station in the minimum time. 
     Details of the assembly machine are better shown in FIGS. 3, 4, 5 and 6 and include some elements omitted in FIG. 2. The base plate 45 has a large central aperture 70 and a short pedestal 72 at each end between the end plates 44. A platen 74 (FIG. 5) is supported on the pedestals with its upper surface flush with the bottom edges of the elements to be loaded into the pallet and is the support for centers when they are first loaded and are not yet held by the adjacent plates. The platen 74 also is used to lift the assembled core out of the pallet via an elevator, not shown, which pushes up through the large aperture 70 in the base plate. 
     The slot 50 in each block is configured to the shape of the plates 12 and 14 so that the plate pairs 10 nest in the slot. The blocks also have relief to accommodate the offset portions 28 of the plates. Each block 48 has, in addition to the slot 50 and the cam follower 52, two flat side faces 75 which abut similar faces in adjacent blocks, two large holes 76 and two small holes 77 receiving the rods 46. The four holes are positioned at corners of a rectangle and two diagonal small holes 77 are surrounded by a boss 78 protruding beyond the faces 75 on one side of the block and containing a bushing 80 for sliding on the rods 46. The other set of diagonal holes 76 are large enough to receive the bosses 78 of the adjacent block. For the blocks to fit together with the adjoining faces 75 in contact, two block types (for each side of the pallet) are used alternately so that each boss 78 of one block will align with and fit in the corresponding large hole 76 of the adjacent block. One type of block rides on two of the rails and the other type rides on the other two rails. The bosses protrude toward the end of the pallet 40 that contains some free space for block movement. The springs 49 on two of the rods reside in the space and extend between the end plates 44 and the bosses 78 of the end block to press the blocks together in the absence of the lead screws. When the lead screws engage some of the blocks the springs 49 are compressed due to the separation of those blocks. The frame 42 is moved by the lead screws 54 via forces acting through the blocks and the springs 49 if the springs are in the leading edge of the pallet. The other end of the pallet may be positioned in the front in which case the force from the lead screws is delivered directly by the blocks to the frame 42. 
     The lead screws 54 comprise helical threads 82 having a pitch determined by the block thickness and the block separation appropriate for a particular loading station. The thickness of each thread is sufficient to span the distance between adjacent cam followers 52. This assures that each block will be positively positioned by the screw threads. The cam followers 52 are essentially elliptical to accommodate the pitch angle of the threads. 
     FIG. 7 shows the lead screw 62 connected to the auxiliary lead screw 66 and coupled respectively to a pallet 40 in the center loading station and a waiting pallet 40&#39;. The screw driving shaft 68 is supported in three spaced bearing blocks 84 and is driven at on end by a motor (not shown). Each screw 62, 66 is mounted on the shaft 68 and keyed thereto by a pin 86 passing through a hub 88 on the screw and through the shaft. The screw 62 is the same as the screw 54 at the plate loading station except for a smaller lead and thread width to conform to the smaller block spacing required for the center insertion. The pallets 40, 40&#39; are shown with one closely following the other. A bumper 90 is fixed to the trailing end of each pallet for desired spacing of the waiting pallet from the one being loaded. This allows the leading blocks of the waiting pallet to smoothly mesh with the screw 66. In operation, the waiting pallet 40&#39; is brought into contact with the rear end of the pallet 40 by the conveyor 58 preferably before the pallet 40 enters the loading station. When the first blocks of the pallet 40&#39; reach the screw 66 the blocks will be captured by the screw so that the pallets 40 and 40&#39; will be advanced together by the screws 62 and 66. When the pallet 40 is fully loaded with centers the motors 64 will accelerate to quickly discharge the pallet 40 and simultaneously move the waiting pallet 40&#39; into the station with accurate positioning for the insertion of the next center dropped into the loading plane. 
     Accurate positioning of the blocks is assured by driving the blocks directly by the lead screws and by driving the lead screws by servomotors under computer control. The amount of rotation of the servomotors and thus the position of each block is precisely controlled by the computer program.